Aquatic fouling organisms such as barnacles, tubeworms, common mussels, Bugula neritina, sea squirts, green laver, sea lettuce, and slimes attach to ships (especially ship bottoms), fishing tools such as fishing nets and fishing net accessories, and structures submerged in seawater such as power plant aqueducts, leading to dysfunction, impaired appearance, and other problems of the ships and so on.
Since a conventional organic tin-containing copolymer has been banned, triorganosilyl group-containing copolymers which have low toxicity and are environment-friendly have been developed and have been used for antifouling coating compositions (Patent Literature 1).
These copolymers can be typically produced by polymerization using a general-purpose polymerization initiator such as t-butylperoxy-2-ethyl hexanoate or AIBN, etc. The copolymers having an weight-average molecular weight (Mw) of 10,000 to 100,000 have been frequently used.
The triorganosilyl ester copolymer produced by polymerization using the above general-purpose polymerization initiator may be used for an antifouling coating composition. In this case, an antifouling coating film formed using the antifouling coating composition dissolves in seawater at a constant rate during the initial stage. However, the dissolving rate of the coating film gradually increases. After a long period has passed, the dissolving rate of the coating film becomes too large. Unfortunately, it is thus difficult to design a coating material. Because of the above, disclosed is an antifouling coating material that exerts the long-term stable dissolution property of the coating film by using rosin, a rosin derivative, or a metal salt thereof in addition to the triorganosilyl ester-containing copolymer (Patent Literature 2).
Although the antifouling coating material can exert the long-term stable dissolution property of the coating film, coating film defects such as a crack occur in the coating film after long-term soaking in seawater. This problem is evident when the low-molecular-weight copolymer, in particular, is used.
So far, a solution containing the copolymer used for the antifouling coating composition has been generally produced using a batch reaction apparatus equipped with a stainless-steel reaction tank, an outer jacket, an internal coil, a condenser, a mixer etc. In this batch reaction apparatus, the temperature is controlled by a coolant that flows through the outer jacket and the internal coil. When the production of the copolymer is continuously repeated, a copolymer-derived gel material insoluble in a solvent attaches to and accumulates in the inside of the reaction tank. Because of this problem, when the copolymer is produced, it takes an extra effort and time to wash the inside of the reaction tank. Consequently, there exists a big problem in the aspect of productivity. Further, it has been difficult to control heat generation during polymerization because the gel material attached to the internal coil causes a decrease in the cooling capacity.
As a method for washing away the copolymer-derived gel material attached, disclosed is a method for washing a batch reaction apparatus by using a high-pressure, highly concentrated alkali (15 to 30% by weight) solution (Patent Literature 3). However, because the highly concentrated alkali is made to contact a stainless-steel reaction tank under high-temperature and high-pressure conditions, the reaction tank is corroded and metal atoms are mixed in a copolymer solution, and thus the copolymer solution is colored. In addition, there is such a problem that because the reaction tank is subjected to successive washing, the reaction tank is damaged and its service life is shortened.
Also, as a method for washing away a gel material derived from a thermoplastic acryl copolymer, disclosed is a method for washing a stainless-steel reaction tank by using a aqueous washing solution containing an alkali metal hydroxide (from 1 to less than 15% by weight) and at least one monohydric alcohol (from 3 to 50% or less by weight; the number of carbons is 2 or 3) (Patent Literature 4). This method, however, is performed under mild conditions such as normal pressure and is yet to provide a complete solution to the problems. Besides, a work step of washing away the gel material is still needed. Consequently, this method does not lead to an increase in the productivity of the copolymer.
Further, as for a method of suppressing generation of a gel material itself, disclosed is a batch reaction apparatus having an external circulation cooler (Patent Literature 5). Although the gel material is not generated on, for example, an internal coil in a reaction tank, the gel material is generated in the external circulation cooler. Consequently, washing work is still needed. This method also does not lead to an increase in the productivity of the copolymer.
As described above, unfortunately, the copolymer produced by polymerization using a commonly used polymerization initiator such as t-butylperoxy-2-ethyl hexanoate or AIBN gives a coating film poor properties such as coating film defects (e.g., cracks) in seawater after a given period of soaking in seawater. In addition, due to the poor coating film properties, the long-term stable dissolution property and antifouling performance of the coating film cannot be maintained. Moreover, there exists another manufacturing problem such as poor productivity caused by the generation of the gel material.